One succesful sparring exercise I've had in prepping my students for upcoming tournaments is this. . . . .

One student stays in the ring. All other students line up outside of it.
Each contender spars the student in the ring for 5-10 secounds giving the "ring defender" a chance to experience everyones explosive first combinations. No pause between contenders. When I say stop, one contender leaves. When I say go the new contender immediately begins.

The "ring defender" gets an exciting workout and a chance to spar everyone in the room.

When the "ring defender" has sparred everyone in the room. The next person in line becomes the new "ring defender".

This drill mirrors an MMA exercise called "the shark tank". .the fighters at my buddy Tony Fryklunds' school (The Militich Academy) in Vegas do this as part of a weekly regimine try changing the duration of the fighting intervals . In JLFS at the blackbelt test a canidate must spar 3-3minute rounds with 1 minute rest intervals .Each round against a fresh fighter(wearing 16 oz gloves )......